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Study of nanostructure growth with nanoscale apex induced by femtosecond laser irradiation at megahertz repetition rate.

Patel NB, Tan B, Venkatakrishnan K - Nanoscale Res Lett (2013)

Bottom Line: We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst.It was found to be possible only in the presence of background nitrogen gas flow.We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Aerospace Engineering, Ryerson University, Victoria Street, Toronto, ON M5B 2K3, Canada. tanbo@ryerson.ca.

ABSTRACT
Leaf-like nanostructures with nanoscale apex are induced on dielectric target surfaces by high-repetition-rate femtosecond laser irradiation in ambient conditions. We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst. It was found to be possible only in the presence of background nitrogen gas flow. In this synthesis method, the target serves as the source for building material as well as the substrate upon which these nanostructures can grow. In our investigation, it was found that there are three possible kinds of nanotips that can grow on target surfaces. In this report, we have presented the study of the growth mechanisms of such leaf-like nanostructures under various conditions such as different laser pulse widths, pulse repetition rates, dwell times, and laser polarizations. We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

No MeSH data available.


Related in: MedlinePlus

Effect of laser pulse repetition rate on plasma expansion and nanotip growth. Nanotips generated for the average laser power of 16 W for pulse repetition rates of (a) 4, (b) 8, and (c) 13 MHz; the dwell time was 0.5 ms.
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Figure 6: Effect of laser pulse repetition rate on plasma expansion and nanotip growth. Nanotips generated for the average laser power of 16 W for pulse repetition rates of (a) 4, (b) 8, and (c) 13 MHz; the dwell time was 0.5 ms.

Mentions: As the repetition rate is decreased, the size of the tips and the number of tips grown varies. These changes in nanostructure can be explained by how the incoming laser pulses interact with target and the plume of ablated species for each repetition rate. High repetition rates provide more pulses hitting the same spot for a given dwell time in comparison to lower repetition rates. In our investigation, the dwell time is 0.5 ms which provided 6,500, 4,000, and 2,000 pulses for repetition rates of 13, 8, and 4 MHz, respectively. The laser power used was on average 16-W which provides the pulse energies of 4.00, 2.00, and 1.23 μJ for 4-, 8-, and 13-MHz repetition rates, respectively. Although the pulse energy (1.23 μJ) and the pulse separation time (77 ns) between two subsequent pulses, as mentioned above, have the smallest value, the heat build-up is the highest for 13-MHz repetition rate in comparison to other two repetition rates. The reason for this is that the plasma created by the previous pulse does not have enough time to relax before the subsequent pulse arrives in the focal region which further heats the plasma species. As a result, for each progressive number of pulses, a much larger volume than the focal volume is heated above the melting temperature of the glass and larger diameter, compared to laser beam spot diameter, of glass melts on the surface due to highly heated plasma and interaction of the laser pulses [23]. Thus, the plume generated at higher repetition rate is much wider and lasts in air for a longer time, as depicted in schematics of Figure 6c. At a higher number of pulse interaction, the vapor distribution inside the plume rapidly loses its symmetry and becomes more and more turbulent [22]. The turbulences at the outer edges of the plasma are generated due to the interactions between incoming nitrogen gas flow and the outgoing plasma species. The turbulence in the core of the plasma results due to the interactions between the highly energized plasma species due to incoming laser pulse absorption and nitrogen gas molecules. Due to the more turbulent interactions and excessive plasma material during 13-MHz repetition rate machining, the plasma species expand wider, and thus, the redeposition back to the target surface occurs over a larger surface area resulting in the formation of a much larger number of randomly oriented leaf-like nanotips, as seen in Figure 6c. When the ablation is performed at the 8-MHz repetition rate, the plasma must have ideal condition in terms of the amount of the turbulence and available ablated material resulting in the growth of highly populated and oriented narrower nanotips compared to 13 MHz, as seen in Figure 6b. For a low number of pulses, the plasma expansion and interaction with surrounding nitrogen gas is less turbulent. The plasma has more time to relax before the next pulse arrives. Thus, the plasma does not expand outward as much resulting in the plasma species being closer. This resulted in the formation of larger droplets of vapor content which get deposited over the target surface area. As a consequence, only a few nanotips are found to be growing randomly from large droplets for the 4-MHz repetition rate, as seen from Figure 6a.


Study of nanostructure growth with nanoscale apex induced by femtosecond laser irradiation at megahertz repetition rate.

Patel NB, Tan B, Venkatakrishnan K - Nanoscale Res Lett (2013)

Effect of laser pulse repetition rate on plasma expansion and nanotip growth. Nanotips generated for the average laser power of 16 W for pulse repetition rates of (a) 4, (b) 8, and (c) 13 MHz; the dwell time was 0.5 ms.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC3643873&req=5

Figure 6: Effect of laser pulse repetition rate on plasma expansion and nanotip growth. Nanotips generated for the average laser power of 16 W for pulse repetition rates of (a) 4, (b) 8, and (c) 13 MHz; the dwell time was 0.5 ms.
Mentions: As the repetition rate is decreased, the size of the tips and the number of tips grown varies. These changes in nanostructure can be explained by how the incoming laser pulses interact with target and the plume of ablated species for each repetition rate. High repetition rates provide more pulses hitting the same spot for a given dwell time in comparison to lower repetition rates. In our investigation, the dwell time is 0.5 ms which provided 6,500, 4,000, and 2,000 pulses for repetition rates of 13, 8, and 4 MHz, respectively. The laser power used was on average 16-W which provides the pulse energies of 4.00, 2.00, and 1.23 μJ for 4-, 8-, and 13-MHz repetition rates, respectively. Although the pulse energy (1.23 μJ) and the pulse separation time (77 ns) between two subsequent pulses, as mentioned above, have the smallest value, the heat build-up is the highest for 13-MHz repetition rate in comparison to other two repetition rates. The reason for this is that the plasma created by the previous pulse does not have enough time to relax before the subsequent pulse arrives in the focal region which further heats the plasma species. As a result, for each progressive number of pulses, a much larger volume than the focal volume is heated above the melting temperature of the glass and larger diameter, compared to laser beam spot diameter, of glass melts on the surface due to highly heated plasma and interaction of the laser pulses [23]. Thus, the plume generated at higher repetition rate is much wider and lasts in air for a longer time, as depicted in schematics of Figure 6c. At a higher number of pulse interaction, the vapor distribution inside the plume rapidly loses its symmetry and becomes more and more turbulent [22]. The turbulences at the outer edges of the plasma are generated due to the interactions between incoming nitrogen gas flow and the outgoing plasma species. The turbulence in the core of the plasma results due to the interactions between the highly energized plasma species due to incoming laser pulse absorption and nitrogen gas molecules. Due to the more turbulent interactions and excessive plasma material during 13-MHz repetition rate machining, the plasma species expand wider, and thus, the redeposition back to the target surface occurs over a larger surface area resulting in the formation of a much larger number of randomly oriented leaf-like nanotips, as seen in Figure 6c. When the ablation is performed at the 8-MHz repetition rate, the plasma must have ideal condition in terms of the amount of the turbulence and available ablated material resulting in the growth of highly populated and oriented narrower nanotips compared to 13 MHz, as seen in Figure 6b. For a low number of pulses, the plasma expansion and interaction with surrounding nitrogen gas is less turbulent. The plasma has more time to relax before the next pulse arrives. Thus, the plasma does not expand outward as much resulting in the plasma species being closer. This resulted in the formation of larger droplets of vapor content which get deposited over the target surface area. As a consequence, only a few nanotips are found to be growing randomly from large droplets for the 4-MHz repetition rate, as seen from Figure 6a.

Bottom Line: We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst.It was found to be possible only in the presence of background nitrogen gas flow.We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Aerospace Engineering, Ryerson University, Victoria Street, Toronto, ON M5B 2K3, Canada. tanbo@ryerson.ca.

ABSTRACT
Leaf-like nanostructures with nanoscale apex are induced on dielectric target surfaces by high-repetition-rate femtosecond laser irradiation in ambient conditions. We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst. It was found to be possible only in the presence of background nitrogen gas flow. In this synthesis method, the target serves as the source for building material as well as the substrate upon which these nanostructures can grow. In our investigation, it was found that there are three possible kinds of nanotips that can grow on target surfaces. In this report, we have presented the study of the growth mechanisms of such leaf-like nanostructures under various conditions such as different laser pulse widths, pulse repetition rates, dwell times, and laser polarizations. We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

No MeSH data available.


Related in: MedlinePlus